1. Field of the Invention
The present invention relates to an all-optical polarization independent optical time division multiplexer and demultiplexer for multiplexing and demultiplexing the modulated optical signal pulses on a time axis by using the optical Kerr effect.
2. Description of the Background Art
Conventionally, there has been a proposition for the all-optical configuration for the multiplexing and demultiplexing of the optical signal pulses on a time axis, using the optical Kerr medium as the optical time division multiplexer and demultiplexer, which utilizes the optical phase shift (optical Kerr effect) induced on the optical signal pulses through the cross-phase modulation by the optical control pulses.
Such a conventional optical time division demultiplexer utilizing the optical Kerr effect is called a nonlinear optical loop mirror, which has a configuration as shown in FIG. 1.
In this configuration of FIG. 1, input optical signal pulses F, which are time-division-multiplexed at a bit rate of Nf.sub.0 (bit/s) where N is an integer which is set equal to 2 in FIG. 1, are entered from an input port 12 of an optical circulator (CIR) 11 and directed from an output port 13 of the optical circulator 11 to an input port 16 of an optical wavelength division multiplexer (WF1) 15. On the other hand, optical control pulses G at a bit rate of f.sub.0 (bit/s), are entered from an input port 17 of the optical wavelength division multiplexer 15. Then, the wavelength-division-multiplexed pulses obtained from the input optical signal pulses F and the optical control pulses G are outputted from an output port 18 of the optical wavelength division multiplexer 15 to a first input port 20 of a 2.times.2 optical coupler (CO) 19. The splitting ratio of the optical coupler 19 depends on the wavelengths of the pulses, so that the optical signal pulses are split at the splitting ratio of 1:1 and outputted from both of output ports 22 and 23 of the optical coupler 19, while the optical control pulses are entirely outputted from one output port 22, i.e., at the splitting ratio of 1:0.
The output ports 22 and 23 of the optical coupler 19 are connected with an optical Kerr medium 24, and the pulses propagated through the optical Kerr medium 24 are outputted from a second input port 21 of the optical coupler 19 to an input port 26 of an optical wavelength division demultiplexer (WF2) 25. Then, the output optical signal pulses I are outputted from a first output port 27 of the optical wavelength division demultiplexer 25 as the switched optical signal pulses (transmitted signals), while the optical control pulses are outputted from a second output port 28 of the optical wavelength division demultiplexer 25.
Here, the output ports 22 and 23 of the optical coupler 19 are connected with the optical Kerr medium 24, so that the optical control pulses entered into the optical coupler 19 propagates through the optical Kerr medium 24 only in a clockwise direction, while the optical signal pulses propagates through the optical Kerr medium 24 in both clockwise and counter-clockwise directions.
In this case, the optical signal pulses propagating in the clockwise direction in overlap with the optical control pulses time-wise will have the phase shift .DELTA..phi. due to the cross-phase modulation. This phase shift .DELTA..phi. can be expressed by the following equation (1): EQU .DELTA..phi.=(2.pi./.lambda.s)L.multidot.2n.sub.2 .multidot.Ic(1)
where n.sub.2 is a nonlinear index coefficient of the optical Kerr medium 24, L is a length of the optical Kerr medium 24, Ic is a peak intensity of the optical control pulses, and .lambda.s is a wavelength of the optical signal pulses.
On the other hand, the optical signal pulses propagating in the counter-clockwise direction will receive the phase shift due to the counter-propagating optical control pulses, so that the phase shift takes a very small value proportional to an average power of the optical control pulses.
Consequently, the phase difference between the optical signal pulses which are propagated through the optical Kerr medium 24 in the clockwise and counter-clockwise directions and wavelength-division-multiplexed at the optical coupler 19 can be set to .pi. and 0, depending on whether the optical control pulses are present or not. At this point, in a case the phase difference is equal to .pi., the optical signal pulses are led to the input port 21 different from the input port 20 from which they have been entered because of the interference effect in the optical coupler 19, whereas they are returned to the input port 20 from which they have been entered.
In other words, among the input optical signal pulses F, only those which had overlapped time-wise with the optical control pulses are outputted to the output port 27 of the optical wavelength division demultiplexer 25 as the switched optical signal pulses (transmitted signals) which are time-division-multiplexed at a bit rate of f.sub.0 (bit/s), while the remaining ones are outputted from the output port 14 of the optical circulator 11 through the optical wavelength division multiplexer 15 as the unswitched optical signal pulses (reflected signals) which are time-division-multiplexed at a bit rate of f.sub.0 (bit/s), so that the function of the optical time-division-demultiplexing can be realized.
In a case of utilizing this nonlinear optical loop mirror configuration of FIG. 1 as the optical time division multiplexer, the first optical signal pulses to be multiplexed are entered from the input port 12 of the optical circulator 11, while the second optical signal pulses to be multiplexed are entered from the output port 27 of the optical wavelength division demultiplexer 25, such that the second optical signal pulses overlap with the optical control pulses time-wise, and the multiplexed optical signal pulses can be obtained at the output port 14 of the optical circulator 11.
Now, in this configuration of FIG. 1, the above equation (1) for expressing the phase shift .DELTA..phi. is valid only when the polarizations of the optical signal pulses and the optical control pulses coincide with each other, and the actual phase shift .DELTA..phi. largely depends on the difference in the polarizations. Consequently, in order to stably operate the nonlinear optical loop mirror configuration of FIG. 1 as the optical time division demultiplexer, it is necessary to put the polarizations of the optical signal pulses and the optical control pulses in coincidence with each other, throughout the entire length of the optical Kerr medium 24. For this reason, conventionally, the optical signal pulses and the optical control pulses have been linearly polarized as shown in FIG. 2, along an identical axis (y axis in FIG. 2).
In addition, it has conventionally been necessary for the optical Kerr medium 24 to be in a form of a polarization maintaining optical fiber having birefringence or an ordinary optical fiber combined with a polarization controller, for the purpose of maintaining the polarization state of the optical signal pulses and the optical control pulses propagated through the optical Kerr medium 24, along a fixed principal axis.
However, in order to operate such conventional optical time division multiplexer and demultiplexer, it has been necessary to detect the polarization state of the input optical signal pulses F and control this polarization state to be in coincidence with that of the optical control pulses G by means of an external circuit.
This requirement, however, presents a particularly difficult problem in the field of the optical fiber communication systems in which the stabilization of the polarization state is quite difficult.